Cryopreservation or cryoconservation is a process whereby cells, whole tissues, or any other substances susceptible to damage caused by chemical reactivity or time are preserved by cooling to sub-zero temperatures. Bringing cells to such low temperatures can result in damage caused by the formation of ice during freezing. To avoid this, cryopreservation typically relies on coating the material to be frozen with a class of molecules termed cryoprotectants or cryoprotective agents (CPAs). More particularly, prior to cryopreservation (CP), the material to be frozen is typically placed into a cryopreservation medium, containing one or more CPAs. A commonly used CPA is dimethylsulfoxide (DMSO). However, DMSO is toxic, and even if the cells are washed to remove the DMSO, it cannot be excluded that some remains around the cells.
Cryopreservation methods are critical in the context of stem cell therapy to allow cells that are harvested or cultured to be maintained until use without loss of quality. A number of publications report side effects in patients receiving cellular transplants as a result of DMSO toxicity. Other cryoprotectants have been investigated, but not one has come out as effective as DMSO, in particular for slow freezing cryopreservation methods. Accordingly, the use of DMSO-based cryopreservation media remain popular in use. Moreover it appears that the efficacy of the cryoprotectant may be dependent on the cell type.
Stem cells are undifferentiated biological cells which are able to renew themselves through mitotic cell division and can differentiate into specialized cell types. Stem cells have a wide (potential) application in the treatment of human and animal conditions. Stem cells are divided into different types based on their potency. Pluripotency refers to the ability of a stem cell to differentiate into cells of any of the three germ layers: endoderm (interior stomach lining, gastrointestinal tract, the lungs), mesoderm (muscle, bone, blood, urogenital), or ectoderm (epidermal tissues and nervous system). The only natural pluripotent cells are embryonic stem cells. Induced pluripotent stem cells (iPSCs) are obtained by artificial expression of certain genes in adult somatic cells which allows the cell to become pluripotent.
Multipotency describes stem cells which have the potential to differentiate into multiple but limited cell types. A hematopoietic stem cell can differentiate into different blood cell types but not into cells of other organs such as brain or bone tissue. The increased level of differentiation in multipotent cells, while limiting its therapeutic application to a disease of the corresponding cell type, is believed to limit the risk of tumor formation.
There is a need for improved cryopreservation media and methods for the cryopreservation of stem cells and/or progenitor cells, in particular for slow-freezing cryopreservation methods.